.alpha.-L-Aspartyl-L-phenylalanine methyl ester (hereinafter referred to as .alpha.-APM) is known as an excellent sweetening agent, and as a process for its production, a process is known which comprises subjecting N-formyl-L-aspartic acid anhydride and PM to a condensation reaction in an organic solvent to form N-formyl-.alpha.-L-aspartyl-L-phenylalanine methyl ester (hereinafter referred to as For-.alpha.-APM) and then removing the formyl group from this reaction product to obtain the desired .alpha.-APM. See G.B. No. 2153365A.
However, in the production examples heretofore known, the desired For-.alpha.-APM is not the sole product but, in addition thereto, its isomer, N-formyl-.beta.-L-aspartyl-L-phenylalanine methyl ester (hereinafter referred to as For-.beta.-APM) is also by-produced, and the ratios of For-.alpha.-APM to For-.beta.-APM (hereinafter referred to as the .alpha./.beta. ratio) are not always satisfactory, because, even when the practical concentrations of the reactants according to G.B. No. 2153365A (0.2.about.0.8 M/l) are employed, the .alpha./.beta. ratios are small (.alpha./.beta. ratio=1.8-3.3), and therefore it could not be regarded as an efficient production process. Indeed, when low concentrations of the reactants according to G.B. No. 2153365A (0.02 M/l) are employed, a considerably high .alpha./.beta. ratio of 5.5 is attained, but large reaction vessels are needed therefor. Large reaction vessels are not practical from the industrial point of view. On the contrary, according to the present invention, the .alpha./.beta. ratios can remarkably be enhanced at the practical concentrations of the reactants.
The by-production of For-.beta.-APM reduces not only the yield of For-.alpha.-APM but also that of the end product, .alpha.-APM, and also this means the contamination of .beta.-L-aspartyl-L-phenylalanine methyl ester (hereinafter referred to as .beta.-APM) into .alpha.-APM, but since .beta.-APM does not exhibit sweetness, .alpha.-APM contaminated with .beta.-APM must be purified to remove the .beta.-APM. Therefore, as described above, an increase in the .alpha./.beta. ratio is believed very advantageous in view that the desired end product .alpha.-APM may be obtained in a good yield.